Taxiing operations on runways that are unpaved or, in general, have roughnesses that could represent critical situations for numerous components of modern aircraft, due to the stresses that these roughnesses transmit to the wheels, shock absorbers and structure of the aircraft as a whole.
In general, the surfaces of runways, especially those of the more important civil airports are made as “level” as possible, controlled and maintained at regular intervals to a level of surface finish considered acceptable by the airport operators. In smaller airports where the take-off/landing runways are built on dirt/gravel, the surface finish does not always have the same degree of levelling. In addition, the length of the runways is such that it is virtually impossible to assume a completely “levelled” surface. The stresses transmitted to the aircraft from taxiing on rough runways must be evaluated both from the viewpoint of the strength limit (i.e. no permanent deformation or breakage must occur due to exceeding the design loads) and from the viewpoint of fatigue on the primary and secondary structures of the aircraft.
In general, when defining the roughness of a runway, it is necessary to distinguish between macro-roughness like bumps or dips, whether located in isolation from one another (for example, located at distances from each other significantly greater than the length of the aircraft), or mutually consecutive (i.e. located close to each other, for example, at a distance approximately half the length of the aircraft or less), and micro-roughness for which one talks more properly of surface roughness. Generally speaking, it can be asserted that surface roughness is the type of asperity that, for the most part, can only cause damage to the tyres of the wheels, first of all causing precocious wear. Instead, the bumps/dips represent variations in runway height, which can be either abrupt or gradual with respect to the measured profile of the runway considered, and which, depending on the aircraft's taxiing speed, the length of the roughnesses and their height, generate stresses that the shock absorbing systems cannot always sufficiently contain. In this case, stresses induced by the above-stated roughnesses are transmitted to the rest of the aircraft's cell that, in addition to reducing passenger comfort, can cause damage both due to exceeding the design limit loads and due to the development of vibrations (in any case, to be investigated to exclude the triggering of resonance phenomena with the structure's intrinsic frequencies), as well as an increase in structural fatigue with respect to the case of using an (ideally) smooth runway.
Generally speaking, the known methods of runway roughness analysis are focused on defining the acceptable level of surface roughness for all commercial aircraft that must perform take-off/landing/taxiing operations on the given runway (in order to assess comfort on board the aircraft and to maximize the working life of the aircraft itself and its components, such as the undercarriage for example). These methods therefore provide a useful evaluation of the runway for airport authorities to control the state and maintenance of the runway.